1. Field of Use
This invention relates generally to liquid level controls which maintain the level of liquid in a vessel within a predetermined range and which perform other safety-related functions if the range is exceeded, such as operation of alarms and pumps.
In particular, it relates to a liquid level control which comprises a float assembly and a switching assembly having magnetically actuatable switches.
The invention is especially well-adapted for use in controlling the level of liquid refrigerant in a vessel, such as a low-pressure receiver, in a large refrigeration system but could have other uses.
2. Description of the Prior Art
Large refrigeration systems typically employ an evaporator, a compressor, a high pressure condenser, a high pressure receiver and a low pressure receiver located between the high pressure receiver and the evaporator. The low pressure receiver receives liquid refrigerant (such as ammonia, Freon, or the like) from the high pressure receiver through a solenoid valve and a manually adjustable expansion valve. Liquid refrigerant accumulates in the bottom of the low pressure receiver and is discharged therefrom to the evaporator through a lower outlet port either by gravity (as in a one-coil flooded evaporator system) or by means of a selectively operable motor-driven pump (as in a liquid overfeed recirculating evaporator system). Evaporated refrigerant rises to the top of the low pressure receiver and is discharged therefrom through an upper outlet port to the compressor. Changing operating conditions in the refrigeration system cause the liquid level in the low pressure receiver to fluctuate and the solenoid valve supplying liquid refrigerant to the low pressure receiver is operated (open or closed) as necessary to prevent the liquid level from dropping below or rising above predetermined low and high levels, respectively. If the liquid level rises too high, there is a danger that the liquid refrigerant will be discharged or backed-up through various ports in the low pressure receiver, as through the upper outlet port of the low pressure receiver and into the compressor, and thereby cause malfunction of or damage to the compressor. If liquid level is too low, the evaporator will not be properly supplied with refrigerant.
Various types of liquid level controls are presently employed to sense liquid levels in the low pressure receiver and to operate the solenoid valve accordingly and to operate an alarm if levels are exceeded. One prior art liquid level control, such as that shown in U.S. Patent 4,647,740, is mounted on the exterior of the vessel and comprises a float assembly and a switching assembly mounted on and above the float assembly. The float assembly comprises a float chamber which is connected by piping to the low pressure receiver so that the level of liquid in the float chamber corresponds to that in the low pressure receiver. The float assembly further comprises a float ball in the float chamber which is buoyantly supported by the liquid refrigerant therein and rises and falls with the liquid level. A float rod connected to the float ball extends vertically into the switching assembly and moves axially up and down along a vertical path as the float ball rises and falls. That portion of the float rod within the switching assembly comprises two axially (vertically) spaced apart stop members. The switching assembly comprises a magnetically attractive actuator in the form of a metal sleeve (which is not itself a magnet) which is slidably mounted on the float rod between the stop members. The switch assembly also comprises a switch unit which includes one or more proximity switches, each electric switch being mounted in a fixed position relative to the float chamber and alongside the aforesaid vertical path. The switch(es) operate equipment and/or alarms. A lever arm is provided for operating each switch and is pivotally mounted relative to the switch. A U-shaped permanent magnet is mounted on the lever arm and is disposed adjacent the path of movement of the float rod and both poles of the permanent magnet simultaneously cooperate with the metal sleeve thereon. A biasing spring is connected to the lever arm to bias the lever arm (and the associated switch) in one switching position but to enable the lever arm to be moved against spring bias to another switching position in response to magnetic attraction between both poles of the permanent magnet and the actuator sleeve. When such magnetic attraction ceases to exist, the spring biases the lever arm (and switch) back to the said one switching position. In one typical prior art arrangement, as the liquid level and float rod descend, the upper stop member eventually causes the actuator sleeve to descend out of the magnetic field of the permanent magnet thereby terminating the magnetic attraction. As this occurs, the spring pulls the lever arm away from the sleeve and causes the switch to be actuated to one position. As the liquid level and float rod rise, the lower stop member eventually causes the sleeve to rise to a position wherein magnetic attraction between both poles of the permanent magnet and the actuator sleeve pulls the lever arm against spring bias and actuates the switch to its other position. Thus, a differential is maintained between acceptably low and acceptably high liquid levels and constant recycling of the solenoid valve in response to departure from a single acceptable liquid level, as in some prior art liquid level controls, is avoided.
Prior art liquid level controls of the aforesaid character have become very complex and expensive. Provision of biasing springs of the aforedescribed character to effect switch operation when magnetic attraction terminates has added to cost and unreliability. Furthermore, because the switch is spring biased to one position, it returns to that position when the sleeve changes position and magnetic attraction ceases. Some more complex arrangements, therefore, require two such switches, one to effect switching and another to maintain a circuit energized (or de-energized) after initial switching occurs. Also, the need to provide high and/or low liquid level alarm switches in addition to those switches which actually effect operation of solenoid valves and pumps has added to complexity and cost. Therefore, there is a need for an improved liquid level control having an improved switching assembly.